Radio receiver



Oct. 20, 1942. R, 5, HQLMES 2,299,391

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N. J., assignor to Radio Corporation of America, a corporation ofDelaware Application May 27, 1941, Serial No. 395,346

(Cl. Z50-20) 6 Claims.

My invention relates to radio receivers and particularly to radioreceivers having noise limiting circuits.

emphasis of these components at the transmitter, the prior art practiceis to have the attenuation circuit at some point following the noiselimiter.

An object of the invention is to provide an improved receiver of thetype including a noise limiter and including, also, means forattenuating pre-emphasized signals and/or means for providing tonecontrol.

I have found that improved receiver operation is obtained by employing anoise limiter of the type having a resistor-capacitor or R-C circuitacross which there appears a gate or limiter voltage which varies withthe envelope of the moduor all of the frequency band width limitation isat the noise limiter itself with the result that there is improved noisereduction.

The invention will be better understood from the following descriptiontaken in connection with "5 the accompanying drawing in which Figure 1is a circuit diagram of one embodiment of my invention applied to thesound channel of a television receiver,

Figure 2 is a group of curves that are referred to in explaining theinvention, and

Figures 3 and 4 are circuit diagrams showing other embodiments of theinvention.

Referring to Figure 1, there is shown at I0 the front or head end of atelevision receiver comprising a first detector and a tunable oscillatorfor passing an intermediate frequency picture signal to a televisionchannel (not shown) and for passing the accompanying I. F. sound signalto the sound channel.

The sound channel includes an I. F. amplifier II which supplies soundsignal through a transformer I2 to a second detector I3. The transformerI2 is tuned to have a Wide pass band cors.

responding to the pass band of This pass band is substantially bandwidth of the signal being ample, the amplifier pass band the amplifierII. greater than the passed. For exmay be 200,000

' average carrier signal cycles wide while the sound signal may have a--f frequency band lying between 60 cycles and 8000 cycles.

The second detector I3 comprises a diode I 6 having a plate connected tothe upper end of the secondary of transformer I2 and having a cathodeconnected through an output resistor I1 to the midpoint of thesecondary.

A noise limiter is provided at the second detector, the limiter in thisparticular example comprising a diode I 8 having its cathode connectedto the lower end of the transformer secondary and its plate connectedthrough a resistor R to the cathode of the diode I6. A condenser C isconnected across the resistor R whereby a gate or limiting voltageappears across the R-C network. The time constant of this networkpreferably is such that the gate voltage follows the carrier waveenvelope representing the useful modulating signal. It would be made tofollow the modulation envelope almost exactly if it were not for thedesired attenuation of pre-emphasized signal components referred tobelow.

The type of noise limiter just described is similar to that describedand claimed in application Serial No. 299,064, led October 12, 1939, inthe names of Ralph S. Holmes and Ciro C. Martinelli, and shown in itsbroader aspects in Gelzer Patent 2,194,499. In this type of limiter, thebasic principle of operation in reducing noise is that if the signal(noise) exceeds a certain limiting amplitude (which amplitude herevaries with the modulation envelope), it then causes current ow in diodeI8 as well as in diode I6 and the noise signal is balanced out. Theapparatus of Fig. 1 differs from that described in the Holmes andMartinelli application in that the R-C circuit is so adjusted that itprovides both noise reduction and the desired attenuation of highfrequency components which have been pre-emphasized at the transmitter.This will be explained later with reference to Figure 2.

The audio signal from detector I3 is supplied to an audio amplifier 2|which may be provided with 'a suitable tone control means as indicatedon the drawing. A loud speaker is indicated at 22.

The receiver preferably includes a suitable automatic volume controlcircuit of the average carrier type. It may comprise a circuit supplyingfrom the detector I3 through the usual filter 23-24 to a D. C. amplifier26. The D. C. output of this amplifier is applied to the I. F. amplifierII to control its gain.

The specic receiver above described is designed to attenuate a soundsignal which has been accentuated or pre-emphasized in accord- Thus allband width ance with the impedance-frequency characteristic of a seriesinductance-resistance network, this complementary attenuation beingprovided by the R-C network of the noise limiter. From a physicalviewpoint, this means that the time constant of the R-C network is suchthat the gate voltage does not follow the envelope of the modulated waveexactly but, instead, digs into the portion of the envelope representinghigh frequency components to prevent the passage of some of the signalas well as the noise.

rIhe attenuation characteristic of the receiver of Fig. 1 is shown inFig. 2 where the amount of pre-emphasis at the transmitter is alsoshown. Preferably, the R-C circuit of Fig. 1 is adjusted to provide a atoverall frequency response for the transmitter and receiver combined.This is assuming the tone control in the amplifier 2| is set to pass allthe frequencies. Or, stated differently, the overall systemcharacteristic preferably is flat up to the tone control circuit ofamplier 2 I.

The above-described receiver design will give better noise suppressionthan can be obtained by adjusting the R.-C. circuit for proper noisesuppression alone and then obtaining the cornplementary high frequencyattenuation by means of a subsequent separate network.

My invention may be carried a step further by locating the tone controlmeans at the R.C. network as shown in Fig. 3, by way of example.limitation for the sound channel is at the noise limiter itself andmaximum eiectiveness of noise-reduction is obtained.

In Figs. 1 and 3, like parts are indicated by the same referencenumerals. In this embodiment the time constantA of the R.C. circuit iseffectively varied for tone control purposes by means of a resistor 3|and a condenser 32 connected in series with each other and across theR.--C. network. A change in the amount of resistance 3l in the circuitchanges the amount of capacity effectively across the resistor R of theR.C.

. network. Thus the gate` voltage may be caused to vary at a rate thatremoves more or less of the high frequency components as desired. Morespecifically, if the tone control is adjusted to` reduce the amount ofhighs in the signal, the overall frequency response of the system willnot be'at as shown in Fig. 2 ,but will droop a certain amount dependingupon the setting of the tone control.Y

The apparatus shown in Fig. 4 is the same as that of Fig. 3 except thatthe tone control means comprises a resistor 34, a condenser 35 and aninductance coil 36 in series with each other and connected across theR.C. network. By changing the value of resistor 34 the effectiveness ofthe series resonant section 35-36 is changed whereby thea'mount ofattenuation of the high frequency components may be varied at will. Theelements 35 and 36 may be tuned for resonance at one of the higherfrequencies of the sound signal band such as 5000cycles per second;

Itmay be noted that theV circuit of' Fig. 3 is preferred as comparedwith that of Fig. 4 since a resistor and condenser in parallel may begiven a characteristic which is exactly complementary tothe seriesresistor-inductance networkrused for pre-emphasis at the transmitterwith which the receiver is to operate.

On the drawing the values of various elements have been indicated,merely by way of example, in ohms and microfarads.

It should be understood that my invention is balanced type of noiselimiter described herein and may be applied to other noise limiters suchas the limiter described in application Serial No. 395,363, filed on thesame day as the present application in the name of Ciro C. Martinelli.

I claim as my invention:

1. A receiver for the reception of a carrier wave modulated by a signalwhich has been preemphasized at a transmitter in accordance with arising frequency-impedance characteristic which is substantially thesame as that of an inductance coil and a resistor in series, saidreceiver including a noise suppressor circuit in which the limitingnoise level is determined by a networkk including a condenser and aresistor in parallel, the capacity and resistance values of saidcondenser and said last resistor being such that the said noisesuppress-or circuit has a falling frequency-impedance characteristicwhich is substantially complementary to said frequency-impedancecharacteristic utilized at the transmitter for pre-emphasis.

2. A receiver for the reception of a carrier wave modulated by a signalwhich has been pref emphasized at a transmitter in accordance with arising frequency-impedance characteristic which is substantially thesame as that of an inductance coil and a resistor in series, saidreceiver including a noise suppressor circuit comprising means forproducing a limiting noise level voltage, said means comprising anetwork including a condenser and a resistor in parallel across which atleast a portion of said voltage appears, the capacity and resistancevalues of said condenser and said last resistor being such that the saidnoise suppressor circuit has a falling frequency-impedancecharacteristic which is substantially complementary to saidfrequencyimpedance characteristic utilized at the transmitter forpre-emphasis.

3. A receiver for the reception cf a carrier wave modulated by a signalthat has been accentuated at a transmitter in accordance with a risingfrequency-impedance characteristic which is substantially the same asthat of an inductance coil and a resistor in series, said receiverincluding a noise suppressor circuit in which the limiting noise levelis determined by a network inluding a condenser and a resistor inparallel, the capacity and resistance values of said condenser and saidlast resistor being such as to provide a falling frequency-impedancecharacteristic which may be adjusted to be substantially complementaryto said frequency-impedance characteristic utilized at the transmitterfor accentuation, and tone control means included in said networkwhereby its timeconstant may be adjusted for tone control purposes.

4. A receiver for the reception of a carrier wave modulated by a soundsignal, said receiver including a noise suppressor circuit in which thelimiting noise level is determined by a network including a condenserand a resistor in parallel across which at least part of a noiseVlimiting voltage appears, the capacity and resistance values of saidcondenser and said last resistor being such that said limiting voltagefollows at least approximately the envelope of said modulated CarrierWave, and tone control means for varying the time constant of saidnetwork for tone control purposes.

5. 'Ihe invention according to claim 4 wherein said tone control meansconsists of a resistor not limited to the and condenser connected inseries with each other and connected in parallel across said network.

6. The invention according to claim 4 wherein the said capacity andresistance values of said network also are such as to provide a fallingfrequency-impedance characteristic which is complementary to thefrequency-impedance characteristic of an inductance coil and a resistorin series utilized at the transmitter for preemphasizing the modulatedcarrier Wave being 5 received.

RALPH S. HOLMES.

